Outline

Background: The Bragg peak of proton treatment provides clear advantage in dose distribution. As the number of fields increases this advantage grows. While present proton delivery systems do not allow gross movements of the beam itself, robotic patient positioning devices do provide possibilities for moving a patient through defined arcs with the beam on. We decided to test this concept using simple gel dosimetry.

Materials/methods: MAGIC gel was prepared using standard reported methods. Batches of gel were prepared in scintillation vials and allowed to cool for twelve hours. The gels were irradiated with protons in range from 10 cGyE to 3000 cGyE. The irradiated vials were imaged using a 1.5 Tesla MRI, T2 weighted images were obtained and calibration curves were generated from this data. Next, glass flask phantoms were filled with Magic Gel and allowed to cool for twelve hours. One flask was used in basic a three-field treatment to show beam paths and intersections. A second flask was centered at room isocenter on a “Micro-Go-Round” turntable and irradiated horizontally with a pristine Bragg peak and a two centimeter aperture while the flask rotated through 360 degrees. Range was adjusted to the center of the phantom. The flask phantoms were imaged using the same MRI techniques used on the vials. To approximate and analyze the proton arc, a treatment plan was generated on a CMS planning system using a circular phantom, eighteen radial field pristine peaks and a two centimeter aperture.

Results: Our calibration curves demonstrate that MAGIC gel is an unreliable quantitative dosimeter. Despite this, relative dosages can be easily visualized with MRI images. The three field treated phantom clearly shows the three intersecting beam paths with good relative intensities. The rotated phantom demonstrates a nearly perfect central sphere of dosage. There was no other detectable dose within the phantom. The approximated treatment plan of the proton arc showed the entrance dose to be less than 1% of the target dose.

Conclusions: MAGIC gel is an unreliable quantitative proton dosimeter but shows accurately the geometry of relative dose distributions for these kinds of demonstrations. Methods of proton arc therapy may have potential for maximizing protons’ therapeutic advantage.